This invention relates to an electro-optic modulator. For example, the electro-optic modulator can be used as a sensor head in an electrical-field sensor for detecting the electrical field surrounding the sensor head.
A known electro-optic modulator is disclosed in JP-A H08-313577, the disclosure of which is incorporated herein by reference. The electro-optic modulator is fabricated in an electro-optic substrate made of LiNbO3. An optical waveguide comprises a pair of optical paths and is formed in the substrate. On or above the optical waveguides, electrodes are arranged to cause refractive index changes in the pair of optical paths in response to electrical fields surrounding the electro-optic modulator. By using the refractive index changes, the electro-optic modulator can detect and convert the electrical fields into light intensity changes.
The substrate material of LiNbO3 also has a pyroelectric effect. The pyroelectric effect of the substrate induces undesirable distribution of electrical charges depending upon the ambient temperature. The undesirable distribution of electrical charges provides undesirable electrical fields and causes significant errors. The detail of the problem caused by the undesirable distribution of electrical charges is described in JP-A H09-197357, the disclosure of which is incorporated herein by reference.
To solve the undesirable distribution of electrical charges, JP-A H08-313577 has proposed applying a conductive resin material onto the electrodes so that a low conductive film or an inferior-conductive film is formed over the electrodes. However, it is difficult to form the low conductive film of uniform thickness to provide the same conductivity over the film.
JP-A H09-197357 has proposed another approach, in which a buffer layer is a mixture layer of transparent insulator material and transparent conductive material. However, it is also difficult for the mixture layer to have a uniform resistance in a plane.